1. Field of the Invention
This invention relates to a method for producing adamantane, more particularly to a method for producing adamantane in an acidic chloroaluminate ionic liquid.
2. Description of the Related Art
Adamantane (tricyclo[3,3,1,13,7]decane) is a colorless, non-toxic, crystalline compound. With a formula C10H16, it is a cycloalkane, and is also the simplest diamondoid. The chemical and physical properties thereof are as follows: melting point of 268° C., density of 1.07 g/cm3, good heat stability, hydrophobicity (due to high symmetry of molecules), ease of sublimation, and good lubricity.
Since hydrogen atoms in adamantane are easily substituted, SN1 nucleophilic substitution reaction and SE2 electrophilic substitution reaction usually occur in adamantane. In particular conditions, adamantane can be converted into various derivatives through skeleton rearrangement, oxidation, or alkylation reaction such that adamantane and derivatives thereof have great potential usage in medicine, textile industry, catalyst, surfactant, photo-sensing material, etc. However, high manufacturing costs make the development of adamantane limited.
In general, conventional methods for producing adamantane include ring-closing method, aluminum(III) trichloride method, zeolite catalyzing method, and superacid method. However, the conventional methods have disadvantages. For example, the ring-closing method has disadvantages of complicated procedure, many undesired side reactions, necessity for protecting functional groups of reactants, long operation time, and low productivity.
Aluminum(III) trichloride method, zeolite catalyzing method, and superacid method all involve isomerization of tetrahydro dicyclopentadiene (THDCPD) into adamantane. In these methods, many side products are also produced along with adamantane such that selectivity for adamantane becomes relatively low.
Moreover, in the aluminum(III) trichloride method, a large amount of tar is produced so that productivity of adamantane is considerably decreased and the procedure for purifying adamantane becomes complicated. Moreover, recycling of AlCl3 is difficult. In addition, before discharging AlCl3, an alkaline solution is used to degrade the same so that a lot of waste is produced, thereby resulting in environmental problems. Although solid acids (e.g., H2SO4-treated Al2O3, Al2O3—SiO2) are proposed to replace AlCl3 for eliminating the environmental problems, they are not popularly employed because of low activity and short lifetime.
In addition, superacid catalysts are proposed for the isomerization reaction. For example, B(OSO2CF3)3—HSO3CF3 is used to catalyze the isomerization reaction at 100° C. Although productivity of adamantane can be up to 47˜65%, high corrosion of the superacid catalyst to the apparatus and immature technology for handling the superacid catalyst make the method unsuitable for industrial use.
Recently, a new kind of solvent, i.e., ionic solvent, has been proposed. The ionic solvents have many advantages. For example, they have no measurable vapor pressure, and hence can emit no volatile organic compounds so that operation thereof is much safer. Moreover, since the ionic solvents are usually not miscible with the reactants and products, recycling thereof and purification of the products become easier. In addition, the pH value thereof can be adjusted by modifying the proportion of components thereof. Specifically, the pH value of the ionic solvent depends on the components and the proportions of the components, in which acidity is determined by anions of the ionic solvent. For example, in the chloroaluminate(III) ionic solvent (composed of AlCl3 and a quaternary ammonium halide/quaternary phosphonium halide), if the molar fraction of AlCl3 is 0.5, AlCl4− (having low acceptance of electron pairs) becomes dominant so that the chloroaluminate(III) ionic solvent serves as a weak Lewis acid. If the molar fraction of AlCl3 is greater than 0.5, Al2Cl7− and Al3Cl10− are dominant so that the chloroaluminate(III) ionic solvent serves as a powerful Lewis acid.
In view of the aforesaid advantages, researches have focused on use of the ionic solvent in manufacturing adamantane. For example, in Petrochemical Technology, 31, vol. 5, 345-348 (2002), a method for producing adamantane in an ionic liquid is proposed, which includes transforming dicyclopentadiene (DCPD) into endo-tetrahydro dicyclopentadiene (endo-THDCPD) in an ionic liquid of 1-n-butyl-3-methylimidazolium chloride-BF4 ([BMIM]Cl—[BF4]) at 90˜150° C. and 1.0˜1.5 MPa, and isomerizing endo-THDCPD into exo-THDCPD and adamantane in an ionic liquid of 1-n-butyl-3-methylimidazolium chloride-AlCl3 ([BMIM]Cl—[AlCl3]) in the presence of hydrogen (4 MPa).